The present invention relates generally to an apparatus and method for preparing an injectable radiopharmaceutical. More particularly, the present invention relates to an apparatus and method for preparing H.sub.2 [.sup.15 O] in an injectable form having application in Positron Emission Tomography (PET).
Through the emission of positrons from radiopharmaceuticals labeled with radioactive isotopes, PET permits imaging and measuring physiological process within the human body. Radioactive isotopes such as .sup.18 F, .sup.11 C, .sup.15 O, .sup.13 N are typically used in labelling radiopharmaceuticals for use in PET. Radioactive isotopes are generated from a target compound selected to produce a desired radioactive isotope when bombarded by high energy particles such as accelerated protons or deuterons from a cyclotron. The half life associated with these radioactive isotopes is very short, typically on the order of minutes. Oxygen-15 possesses a half-life of 2.04 minutes.
Oxygen-15 labeled water (H.sub.2 [.sup.15 O]) is one of the most widely used radioactive isotopes in PET for assessing regional cerebral blood flow. There are several methods for producing H.sub.2 [.sup.15 O].
H.sub.2 [.sup.15 O] can be produced from either a palladium or platinum catalyzed reduction of Oxygen-15 labeled oxygen gas with hydrogen at elevated temperatures. Another method in which H.sub.2 [.sup.15 O] can be produced is by an isotopic exchange between Oxygen-15 labelled carbon dioxide gas and water. This exchange is accomplished by continuously recirculating a gas flow containing Oxygen-15 labeled carbon dioxide through 8 to 10 milliliters of saline in a bubbling bag, and back into the irradiation target for further irradiation. A peristatic pump is used to recirculate the gas during this process. H.sub.2 [.sup.15 O] can also be produced directly by the recoil production via in-target reaction of .sup.15 O atoms generated by an .sup.16 O(p,pn).sup.15 O reaction on natural abundance water.
Still another method that H.sub.2 [.sup.15 O] can be produced is by the recoil production via in-target reaction of .sup.15 O atoms generated by a .sup.14 N(d,n).sup.15 O reaction on a nitrogen (N.sub.2) gas and hydrogen (H.sub.2) gas target. Most of the nitrogen does not experience a nuclear reaction to form .sup.15 O. The .sup.15 O that is produced combines with the hydrogen to form H.sub.2 [.sup.15 O]. In addition to producing H.sub.2 [.sup.15 O], other nuclear reactions occur to produce impurities. Specifically, a small amount of .sup.13 N and .sup.11 C are produced from the .sup.14 N(d, dn).sup.13 N and .sup.14 N(d, .alpha.n).sup.11 C reactions, respectively. However, the most abundant impurity produced is ammonia (NH.sub.3) gas. This impurity is produced as a consequence of in-target radiolysis, not as a result of a nuclear reaction. That is to say, the nitrogen gas and hydrogen gas ionizes and undergoes a radiolytic reaction to form ammonia gas.